hiatari ga ii heya ha huyu de mo atatakai desu.

A helpful way to break it up is:

• 日当たりがいい = gets good sunlight / has good sun exposure
• 部屋 = room
• 部屋は = as for the room / the room
• 冬でも = even in winter
• 暖かいです = is warm

So the structure is:

• [descriptive clause] + noun + は + rest of sentence

In other words:

• 日当たりがいい部屋 = a room that gets good sunlight
• 日当たりがいい部屋は冬でも暖かいです = A room that gets good sunlight is warm even in winter.

Japanese often puts the description before the noun it modifies.

日当たり means exposure to sunlight or how much sun something gets.

It comes from:

• 日 = sun
• 当たり = hitting / exposure

So 日当たりがいい means something like:

• has good sun exposure
• gets a lot of sunlight
• is sunny

For rooms, houses, and apartments, this is a very common word.

Here, が marks the subject inside the descriptive clause 日当たりがいい.

That clause works like:

• 日当たりがいい = the sun exposure is good

Then that whole clause modifies 部屋:

• 日当たりがいい部屋 = a room whose sun exposure is good

A native English speaker may want to say something like sunny room, but in Japanese it is often built as a full mini-sentence before the noun.

So:

• 日当たり = subject of いい
• が = marks that subject

Yes. 日当たりのいい部屋 is very common and very natural.

Both can be understood, but there is a slight difference in feel:

• 日当たりのいい部屋 = a common set expression, very natural as a noun modifier
• 日当たりがいい部屋 = also understandable and grammatical, with a stronger feeling of a full clause: a room whose sun exposure is good

So if you have seen 日当たりのいい部屋 before, that is normal. The sentence you were given uses が, which highlights the clause structure a bit more clearly.

Because Japanese puts relative clauses before the noun.

English:

• a room that gets good sunlight

Japanese:

• 日当たりがいい部屋
• literally: gets-good-sunlight room

This is completely normal in Japanese. There is no relative pronoun like that, which, or who.

So instead of saying:

• the room that is sunny

Japanese simply puts the descriptive clause directly in front:

• 日当たりがいい部屋

は marks 部屋 as the topic of the sentence.

So:

• 日当たりがいい部屋は = as for rooms that get good sunlight...

Then the speaker says something about that topic:

• 冬でも暖かいです = they are warm even in winter

This gives the sentence a structure like:

• As for sunlit rooms, they are warm even in winter.

Here, は is mainly marking the topic. Depending on context, it can also carry a slight contrastive nuance, like:

• Rooms with good sunlight are warm, unlike some other rooms.

But that contrast does not have to be strong.

冬でも means even in winter.

Here, でも adds the meaning of even.

So the idea is:

• 冬でも暖かい = warm even in winter

That matters because winter is the season when you would normally expect a room to be cold. Using でも emphasizes that the room stays warm despite it being winter.

A natural English gloss is:

• even in winter
• during winter too

Time words in Japanese often do not need に, especially broad time expressions like:

• 今日 = today
• 明日 = tomorrow
• 夏 = summer
• 冬 = winter

So Japanese often says:

• 冬暖かい would sound incomplete here, but
• 冬でも暖かい is natural

In this sentence, it is easiest to treat でも as the important piece: even in winter.

So do not expect Japanese to always match English prepositions like in, at, or on.

暖かい is an -i adjective, and it can be a predicate by itself:

• 暖かい = is warm

Adding です makes it polite:

• 暖かいです = is warm (polite)

Important point:

• Here, です is not changing 暖かい into a noun or na-adjective.
• It is simply making the sentence polite.

So:

• 暖かい。 = plain
• 暖かいです。 = polite

This is a very common pattern with -i adjectives.

いい and よい are forms of the same word, meaning good.

• よい is the dictionary/base form
• いい is the very common everyday form

So:

• 日当たりがいい = natural everyday Japanese
• 日当たりがよい = also correct, but a bit more formal or written-sounding

One thing learners should remember: this word is somewhat irregular in conjugation.

For example:

• present: いい
• past: よかった
• negative: よくない

So it does not become いかった or いくない.

Yes. 暖かい is used for pleasant warmth, especially things like:

• weather
• sunlight
• rooms
• clothing
• atmosphere

So it fits very naturally with a room warmed by sunlight.

It suggests a comfortable, gentle warmth rather than something like intense heat.

That is why 日当たりがいい and 暖かい go together very naturally.